linux-kernel - Re: [PATCH v2] mm/folio: Avoid special handling for order value 0 in folio_set

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <ZGJo4UhdyakGFwP7@casper.infradead.org>
Date:   Mon, 15 May 2023 18:16:17 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Tarun Sahu <tsahu@...ux.ibm.com>
Cc:     linux-mm@...ck.org, akpm@...ux-foundation.org,
        muchun.song@...ux.dev, mike.kravetz@...cle.com,
        aneesh.kumar@...ux.ibm.com, sidhartha.kumar@...cle.com,
        gerald.schaefer@...ux.ibm.com, linux-kernel@...r.kernel.org,
        jaypatel@...ux.ibm.com
Subject: Re: [PATCH v2] mm/folio: Avoid special handling for order value 0 in
 folio_set_order

On Mon, May 15, 2023 at 10:38:09PM +0530, Tarun Sahu wrote:
> @@ -1951,9 +1950,6 @@ static bool __prep_compound_gigantic_folio(struct folio *folio,
>  	struct page *p;
>  
>  	__folio_clear_reserved(folio);
> -	__folio_set_head(folio);
> -	/* we rely on prep_new_hugetlb_folio to set the destructor */
> -	folio_set_order(folio, order);
>  	for (i = 0; i < nr_pages; i++) {
>  		p = folio_page(folio, i);
>  
> @@ -1999,6 +1995,9 @@ static bool __prep_compound_gigantic_folio(struct folio *folio,
>  		if (i != 0)
>  			set_compound_head(p, &folio->page);
>  	}
> +	__folio_set_head(folio);
> +	/* we rely on prep_new_hugetlb_folio to set the destructor */
> +	folio_set_order(folio, order);

This makes me nervous, as I said before.  This means that
compound_head(tail) can temporarily point to a page which is not marked
as a head page.  That's different from prep_compound_page().  You need to
come up with some good argumentation for why this is safe, and no amount
of testing you do can replace it -- any race in this area will be subtle.